1. Field of the Invention
The present invention relates to an improvement on a focus detecting device that is adapted to be equipped on a digital camera or the like and which time-sequentially divides the photographing light beam passing through an optical system into at least two different areas, converts optical images, time-sequentially focused on image sensor means through respective areas, into image signals and detects the phase difference of the image signals thereby calculating the focus state of the optical system.
2. Related Background Art
Among the automatic focusing devices employed in the conventional camera that uses silver halide-based film of the single lens reflex type, there are widely employed those base on the phase difference detecting system.
FIG. 48 is a view showing the arrangement of the optical system of a single lens reflex type, provided with a conventional focus detecting device of the phase difference detecting system, wherein a light beam 9a emerging from a photographing lens 1 is partly reflected by a main mirror 2, consisting of a half mirror, as a light beam 9b toward a focusing screen 3 and focuses an image of the object on a matted face thereof. The photographer observes the object image on the focusing screen, through eyepiece lenses 5a, 5b and a pentagonal roof prism 4.
On the other hand, among the light beam 9a, a part 9e transmitted by the main mirror 2 is reflected by a sub mirror 6 and is guided as a light beam 9f to a focus detecting device 7, which detects, by means of the light beam 9f, the focus state (defocus amount) of the photographing lens 1 relative to a silver halide-based photographic film 8.
The detected defocus amount is larger than a predetermined range of focus so that the photographing lens is judged as in an out-of-focus state, and an unrepresented control circuit so drives a focusing lens of the photographing lens 1 as to cancel the defocus amount, thereby performing the focusing operation.
In the following there will be explained, with reference to FIGS. 49A to 49C, the focus detecting principle of the conventional focus detecting device.
FIG. 49A shows an in-focus state, wherein light beams 16a, 16b passing respectively through two different pupils of a photographing lens 10 are focused on a primary focal plane 14, and images of the object on such primary focal plane are refocused by secondary imaging lenses 12a, 12b on a sensor plane 13 having two line sensors for each of the refocused images. A field lens 11 is provided in the vicinity of the primary image plane of the photographing lens 10, thus efficiently guiding the light beam of a predetermined image height to the sensor plane 13 and preventing the loss in the amount of light resulting from the increase in the image height. The two light beams 16a, 16b, passing through the different pupils of the photographing lens 10, are in general limited by unrepresented diaphragms positioned immediately in front of or immediately behind the secondary imaging lenses 12a, 12b, and the photographing lens 10 is not provided with a member for dividing the pupil. Assuming that xcex40 is the relative distance (phase difference) of the positions of the two images in the in-focus state, the amount of defocus in the current state and the direction thereof can be known from the difference between xcex40 and the actually given phase difference.
FIG. 49B shows a state in which the lens is focused in front by a defocus amount d1, wherein the phase difference xcex41 of the two images becomes smaller than xcex40, and the difference (xcex40xe2x88x92xcex41) increases with the increase of d1.
FIG. 49C shows a state in which the lens is focused in the back by a defocus amount d2, wherein the phase difference xcex42 of the two images becomes larger than xcex40, and the difference (xcex42xe2x88x92xcex40) increases with the increase of d1. In this manner the detection of the phase difference of the two images focused on the sensor plane 13 allows the camera to detect the focus state of the photographing lens, or the magnitude and the direction of the defocus amount.
However, in the prior art, the single lens reflex camera becomes inevitably bulky for securing the space for the focus detecting device and becomes expensive by the cost required therefor. Also, the accuracy of focusing is deteriorated as the relative positional relationship between the photographic film and the focus detecting device is varied by a change in the temperature or by a time-dependent variation in the quick return mirror.
The image taking apparatus, such as a digital camera, employ a solid-state image pickup device as the image sensor and can avoid the above-mentioned drawbacks by employing such solid-state image pickup device as the focus detecting sensor. A method for utilizing the solid-state image pickup device for the focus detecting sensor, for example employed in video cameras, consists of effecting the focusing operation, based on the detected contrast of the object image on the solid-state image pickup device, but such method is incapable of high-speed and highly precise focusing, as the exact defocus amount cannot be detected.
The highly precise high-speed focusing, without the above-mentioned drawbacks, can however, be achieved by providing the photographing lens with pupil dividing means for rendering transmissive either one of the two different pupil areas, and detecting the relative positional difference, or the phase difference, between the object image obtained by the light beam transmitted by such transmissive pupil area and an object image obtained by the light beam transmitted by the other pupil area, thereby detecting the defocus amount.
However, in such conventional configuration, the image signal of the object image formed by the light beam transmitted by a pupil area is stored at first and then that of the object image formed by the light beam transmitted by the other pupil area is stored later, so that the two image signal storage operations are mutually different in time. If the object image on the image pickup sensor moves in a period from the storage of the first image signal to that of the second image signal, there will result a detection error because of such image movement.
This drawback will be explained with reference to FIGS. 13A and 13B, which illustrate a case where the object image on the image pickup sensor moves in the vertical direction by a hand vibration or by the movement of the object itself.
If the camera is inclined downwards by a hand vibration after the storage of a first object image L shown in FIG. 13A, the object image R to be stored next moves upwards as shown in FIG. 13B. In such case, the image signal Rc of an image pickup position the same as that for the image signal Lc, is in fact generated by looking at the object from a lower position, and therefore the camera assumes the image signals indicate a different shape. The correlation calculation utilizing the image signals of different forms leads to an erroneous phase difference.
In this manner the vertical movement of the object image on the image pickup element, caused by hand vibration or by movement of the object, will result in an error in the detected phase difference.
One aspect of the application is to provide an apparatus capable of determining the light receiving position of each image in the vertical direction of an image pickup element (sensor means) according to the vertical moving position of the image on the image pickup element, storing image signals in such determined positions, and detecting the phase difference between thus fetched image signals.
One aspect of the application is to provide a focus detecting device provided with image pickup means for taking an image; an image taking optical system; pupil shape changing means for time-sequentially dividing an image taking light beam, transmitted by the image taking optical system, into at least two different areas and focusing the divided light beams on the image pickup means; and calculation means for converting the optical images, time-sequentially formed on the image pickup means respectively through the different areas, into image signals and calculating the focus state of the image taking optical system by detecting the phase difference of the image signals, the device comprising movement amount detecting means for detecting the amount of movement of the optical image on the image pickup element, in a direction perpendicular to the direction of arrangement of the above-mentioned areas (for example in the vertical direction of a camera), and image position changing means for changing the position of the image signal to be employed in the calculation of the phase difference to be executed in the calculation means.
One aspect of the application is to provide an apparatus for calculating a correlation value indicating the level of coincidence of time-sequentially obtained two images by a correlation calculation, determining the positions of the image signals showing a high level of coincidence based on such correlation value and calculating the phase difference utilizing the image signals of such positions.